The invention relates primarily to so called radiation surgery where radiation beams, typically gamma radiation, are focused towards a limited area, a so called radiological focus, inside tissue. Through the focusing of the radiation beams, a high radiation dose can be achieved in the radiological focus such that the tissue in this focus area can be destroyed. In this way it is possible to destroy for example cancerous tumors inside the brain without having to open the skull.
However, the invention is applicable to any radiation treatment where some kind of focusing of the radiation beams are involved, for example radiation therapy where the main purpose is not to destroy the tissue immediately but through repeatedly treatments gradually deplete the affected tissue. The invention also relates to radiation surgery or therapy of other parts of the body than the brain, though the brain is particularly well suited for this kind of treatment, since it is fixedly positioned in relation to the skull which makes it possible to perform the treatment with high precision. The organs in for example the trunk, on the contrary, are more movable in relation to the skeleton, which makes it more difficult to perform this kind of radiation treatment with high precision. Fixation of affected tissue in relation to treatment equipment is more simple in for example an arm, a leg or in the vicinity of the spinal column where it is possible to secure the tissue in relation to the equipment via the bones.
Consequently, the invention is applicable to all radiation treatment where some form of focusing of the radiation beams is involved. Under these circumstances arises often a need for displacement of the patient or the body part within the treatment volume inside the radiation unit to position the target area to be treated in the radiological focus. The radiation unit may have many different shapes and normally has the form of a bowl or sphere with a treatment volume formed like a cavity, or a ring having a circular, square or other form where the treatment volume merely takes the form of a through bore. The body part to be treated is consequently inserted into the treatment volume and displaced until the target area to be treated is in the radiological focus. During the treatment process it might even be necessary to displace the body part to achieve a regular and comprehensive radiation of the hole affected area. During this displacement of the body part, it is always a risk that the body part or any equipment connected thereto, e.g. a fixation device for fixation of the body part, will collide with the interior of the radiation unit with a risk for injury of the patient or damage of the equipment as a consequence.
For example when radiation surgery is performed on a brain, the skull is secured to a fixation frame, a so called stereotactic frame, by means of fixation pins which are threaded into the skull of the patient. The stereotactic frame is in turn secured to a displacement device in the radiation device during diagnosis and treatment. In this way the brain can be positioned in a coordinate reference system and the travel path during treatment can be calculated in advance with great accuracy. The displacement during treatment is normally performed automatically by means of electrical motors, on the one hand because of avoiding exposing the medical personnel for excessive radiation, but also to be able to carry out the treatment with sufficient accuracy in terms of travel path as well as travel rate and dwell time.
In the prior art it is known to detect possible collision by measuring the power supplied to the motors. When the power feed exceeds a predetermined value, the power feed is interrupted and the motors stops accordingly. One problem with such a solution is that the power supply break point can not be set to low because this could lead to an inadvertent interruption due to a natural interference or a heavy patient/body part. Setting the break point to high, on the other hand, could lead to damage of equipment, dislocation of the body part in respect of the coordinate reference system, or even to injury of the patient, in case of a collision. When performing radiation surgery of the brain, it is possible not only to displace the head of the patient during treatment, but at the same time displace the hole body of the patient in order to avoid tensions in the equipment as well as to avoid discomfort for the patient. When displacing the entire body, a considerably larger force is required than when displacing only a part of the body, e.g. the head. Consequently, the break point must be set correspondingly higher which, in case of a collision, will lead to more serious damage until the motors stops.